Motors often have problems with exciting forces in their rotating direction (hereinafter provisionally referred to as “rotating exciting forces”). The rotating exciting forces are broadly divided into cogging torque with no current applied, and torque ripple with a current applied.
It is known that when N denotes the number of slots and P denotes the number of poles (N and P are positive integers), the order of harmonics of cogging torque is the lowest common multiple of N and P.
For example, a motor with 8 poles and 12 slots and a motor with 10 poles and 12 slots are compared in terms of the order. The lowest common multiple of 8 and 12 of the former is 24, whereas the lowest common multiple of 10 and 12 of the latter is 60. Since the order of the cogging torque per turn of the rotor of the motor is larger in the latter, the peak value of the cogging torque is reduced in the latter.
As such, motors with (12±2)n poles and 12n teeth (hereinafter provisionally referred to as “12-slot series motors”) are recognized as promising motors with little vibration and noise. Particularly, fields where smooth torque transfer is necessary (e.g., EPS for vehicles and fan drive motors) require lower rotating exciting forces, and thus the 12-slot series motors are used in these fields.
However, since the winding directions and connection of concentratedly-wound armatures in the 12-slot series motors are more complicated than those of the motors with 8 poles and 12 slots, a problem with deteriorating industrial productivity is known.
In order to address this problem, in Japanese Patent Application Laid-Open No. 2010-193675 indicated below, two connection nozzles are provided per phase, by which respective armature windings are wound parallel in opposite directions. Accordingly, the armature windings can be wound with the connection nozzles operated in the same direction, thus indicating that Japanese Patent Application Laid-Open No. 2010-193675 discloses a technique for increasing the productivity.